About 4 weeks ago I replaced the original battery with an OEM AGM one at $293 from the local BMW dealer. I did the installation and coding/registration myself at home.
Within days after the install I went on a 10 day trip (total 1,300 miles) and about 300 miles from home on my return the CEL came on and stayed on. Back home I read out the error file and it had registered a 2DED Powermanagement error. Usually that points to a disconnected battery. I just cleared the error and no problem since. I am assuming that it had something to do with when I disconnected the battery to put the new one in.
Before buying the new battery I spent hours researching this subject. The AGM battery is now the only type available from BMW, and if you can get it for less than $293, good for you. Also. coding and registering this new type battery is required. A charge in the neighborhood of $100 seems OK, but I have heard of some crazy numbers. So shop around.